In recent years, a solar cell that utilizes clean energy has attracted attention, and various solar cell modules have been developed and proposed. A solar cell module that utilizes a monocrystalline or polycrystalline silicon-based element (device), a solar cell module that utilizes a thin-film amorphous silicon-based element, and the like have been the mainstream. As illustrated in FIG. 1, a solar cell module that utilizes such a silicon-based element typically includes (from the sunlight incident side) a transparent front substrate 1 (e.g., glass substrate), solar cell elements 2, a sealing material 3 that seals the solar cell elements 2, a tab wire 4 connected to the solar cell elements 2, and a back-side protective sheet 5.
A sealing material (EVA sealing material) that is obtained by crosslinking and curing an ethylene-vinyl acetate copolymer (EVA) using a crosslinking agent (e.g., organic peroxide), and exhibits excellent transparency and light resistance, has been widely used as the sealing material 3 that seals the solar cell elements.
However, since EVA includes a structural unit derived from vinyl acetate, EVA is easily hydrolyzed with the passage of time due to moisture and water that have entered from the outside to produce acetic acid. Acetic acid thus produced may come in contact with the wire and the electrode provided inside the cell, and accelerate corrosion.
In order to improve the durability of the solar cell by preventing a decrease in insulating properties due to entry of water into the solar cell, corrosion of the wire and the electrode, and the like, a method that stacks an inorganic oxide deposited layer on the back-side protective sheet as a barrier layer (e.g., Patent Document 1), a method that utilizes a cycloolefin-based resin sheet having a low water vapor transmission rate as the back-side protective sheet (e.g., Patent Documents 2 and 3), and the like have been proposed.
However, it is difficult to completely prevent entry of moisture and water into the solar cell using these methods. Therefore, a sealing material that exhibits low hygroscopicity and low water vapor permeability, and does not produce an acid due to hydrolysis, has been desired in order to remarkably improve the durability of the solar cell by preventing a decrease in insulating properties, and occurrence of corrosion.
A method that utilizes a sealing resin composition that includes an ethylene-α-olefin copolymer and a crosslinking agent instead of EVA (Patent Documents 4 and 5), a method that utilizes a propylene-based polymer and a specific propylene-based copolymer (Patent Document 6), and the like have been proposed in order to solve the problem in which the sealing material produces an acid due to hydrolysis. It is considered that the method that utilizes an ethylene-α-olefin copolymer, and the method that utilizes a propylene-based polymer and a specific propylene-based copolymer ensure that the resin exhibits low hygroscopicity and low water vapor permeability, and reduce the effects of production of an acid.
However, since the balance between the heat resistance and the flexibility of the resin is poor, and excellent heat resistance is not achieved in a state in which the resin is not crosslinked, it is difficult to produce a solar cell element sealing sheet having the desired properties without performing a crosslinking step.
Patent Document 7 discloses a hydrogenated block copolymer that is produced by introducing an alkoxysilyl group into a hydrogenated block copolymer that is obtained by hydrogenating 90% or more of the unsaturated bonds of a block copolymer that includes at least two polymer blocks [A] and at least one polymer block [B], the polymer block [A] including a repeating unit derived from an aromatic vinyl compound as the main component, the polymer block [B] including a repeating unit derived from a linear conjugated diene compound as the main component, and the ratio (wA:wB) of the weight fraction wA of the polymer block [A] in the block copolymer to the weight fraction wB of the polymer block [B] in the block copolymer being 20:80 to 60:40. Patent Document 7 states that the hydrogenated block copolymer disclosed in Patent Document 7 exhibits low hygroscopicity, non-hydrolyzability, weatherability, transparency, and flexibility, maintains high adhesion to glass even when subjected to a high-temperature/high-humidity environment for a long time, and can seal a solar cell element without applying a special waterproof treatment.
A polyvinyl butyral-based resin has been most widely used as an intermediate layer for laminated glass. However, the polyvinyl butyral-based resin has problems in that the glass sheet may be displaced, or air bubbles may occur after bonding due to heat since the polyvinyl butyral-based resin has a relatively low softening point, whitening may gradually occur from the peripheral area, and adhesion to glass may decrease when the laminated glass is subjected to a high-humidity atmosphere for a long time since the polyvinyl butyral-based resin has high hygroscopicity, and it is necessary to strictly manage the water content before bonding glass sheets in order to control adhesion to glass (e.g., Non-patent Document 1), for example.
In order to solve the above problems, Patent Document 8 proposes laminated glass that is obtained by providing a thermosetting resin obtained by adding an organic peroxide to an ethylene-vinyl acetate copolymer, between glass sheets, and thermally curing the thermosetting resin (thermosetting resin layer), and Patent Document 9 proposes laminated glass that is obtained by bonding glass sheets using an acid-modified saponified ethylene-vinyl acetate copolymer, for example. Since sufficient impact resistance and penetration resistance may not be achieved using an ethylene-vinyl acetate copolymer, Patent Document 10 proposes producing laminated glass using a thermosetting resin obtained by adding an organic peroxide and a silane coupling agent to a ternary block copolymer that includes end blocks formed of a polymer of an aromatic vinyl compound, and a middle (intermediate) block formed of a conjugated diene-based polymer, for example. However, since a thermosetting resin that includes an organic peroxide has inferior long-term storage stability, the thermosetting resin may not exhibit sufficient adhesion when bonding glass sheets depending on the storage conditions before bonding glass sheets.
Patent Document 11 discloses providing adhesion to a hydrogenated block copolymer that includes an aromatic alkene polymer block and a diene polymer block by adding an aliphatic hydrocarbon resin or the like. However, Patent Document 11 is silent about providing the hydrogenated block copolymer with adhesion to glass and the like.